Read this page. These are the two main concepts you need to understand after reading this article:
Practice applying these concepts by clicking on the "new problem" button at the end of the article, trying a problem, and checking your answer. Attempt to solve 10 problems. You can also create a worksheet of 10 problems by clicking on the button at the bottom of the page titled "Click Here for a Randomly Generated Worksheet and Answers." The answers will be provided at the end of the worksheet.